Ok, I finally found the problem. During all the rigamorole in dealing with the MEGA boards serial troubles, I had downloaded the latest drivers and some of the files had gotten extracted into the MPIDE folder. When removing all those files, I got a few too many, including rxtxSerial.dll. I unzipped the download into a new folder, and noticed the new files there. Copied them back into the original, and now I am up and running again!